This invention relates to a method for forming fine grain structure compositions for metals and/or ceramics and to the compositions produced.
It is well known that the mechanical properties of solids are controlled by their microstructures. For this reason, metals are often wrought from ingots and machined, rather than being cast into final shapes. Hot and cold working breaks down the carbides in steel and elongates grains which, upon annealing, yield smaller microstructures with small grains and second phase particles. By these means, toughness and strength are imparted to metal parts. However, there are limits to the grain size that can be attained by these techniques. To obtain grains on the order of micron or less, metals and ceramics are produced in fine powders first and then are compacted and sintered to make the final parts. Again, there are limitations in producing parts by powder metallurgy techniques due to the grain growth during sintering and due to oxidation of the powder. The need to work and cut metals and the necessity to produce powder add to the overall manufacturing costs. For example, in the manufacturing of an aircraft spar, a worked and heat treated aluminum ingot is machined on a 3 to 7 million dollar spar mill over many hours to remove more than 70 % of the original aluminum. The manufacturing cost can be reduced substantially if it can be cast to near net shape. There are also many machine parts forged and then machined such as landing gears and cranks, all adding to the cost. Presently, the most expensive process may be a powder metallurgy technique for most parts. Powder metallurgy products cost as much as ten times cast products, although powder metallurgy parts have better mechanical properties and processability than cast products.
The major shortcoming of the casting technology is the lack of the grain size control. Present casting methods can not yield micron sized grains. Although such a fine-grained structure may not be necessary in some applications, there are many other applications where such a uniformly distributed grain structure can substantially enhance the mechanical properties and thus expand the applicability of various materials. Trial products produced from such materials can be shaped more easily and can be made stronger and tougher as compared to products made by present casting techniques. Conventional manufacturing process can only yield these desirable properties at high costs and they are limited by equilibrium thermodynamic considerations.
Most of the modern metal processing techniques depend on equilibrium thermodynamics, except perhaps the recently developed splat cooled powder metallurgy techniques. However, if one can process immiscible materials and yield uniformly distributed phases with a well controlled microstructure, many interesting desirable properties can be imparted to the part. For example, magnetic materials can be dispersed in a non-magnetic matrix and vice-versa. Bearing materials can be made in which the molecules or elements that reduce the coefficients of friction can be incorporated in a matrix. Even such hard particles as oxides, nitrides, borides and carbides can be dispersed in a metal matrix. What is needed is a method of creating solids with various materials combinations and certain desired microstructures.